Far ultraviolet light source



m ,491 i CROSS REFERENCE SEARCH m flfia p iztfxfii June 27, 1:101 c. F. HENDEE r:rA|. 2,990,491

FAR ULTRAVIOLET LIGHT SOURCE Filed July 29, 1958 1940/0067! YE TR! 770M 645 mmvroxs. CFHENDEE BY flFlNf P W E.

Aaflrz 2990491 R IN:.313/5q United States Patent 2,990,491 FAR ULTRAVIOLET LIGHT SOURCE Chrles F. Heudee, Hartsdale, and Samuel Fine, New

York, N.Y., ors to North American Phillips Company, Inc., New York, N.Y., a corporation of Delaware 7 Filed July 29, 1958, Ser. No. 751,747 7 Claims. (Cl. SIS-54) This invention relates to ultraviolet light sources, and in particular to devices for enerating far ultraviolet rg diation with a wavelength of a on 'ngstrom It has been determined that air is substantially transparent to radiation with a wavelength of about 1216 A., but that water is highly absorbent of such radiation. These properties form the basis for a system for determining the moisture or water-vapor content in the air. The system comprises a source of the 1216 A. radiation arranged to traverse a sample of the air with said radiation, which is then detected-in a photon Geiger counter sensitive to such radiation. Such a detector is described in US. Patent No. 2,715,195 when fitted with a suitable window permeable to the 1216 A. radiation, and a suitable gas fill, such as nitric oxide. The greater the water content of the air the more this wavelength of light will be attenuated and the lower will be the counting rate of the Geiger detector. By comparison of its counting rate withonly low-energy beta particles which are capable of ex-.

citing hydrogen or tritium atoms present in the gas from dissociated tritium molecules or separately provided, with the consequence that the excited hydrogen or tritium atoms emit a series of radiations which include that at a wavelength of 1216 A.

The invention will now be described in 'greater detail with reference to the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of one form of the far ultraviolet light source of the invention;

FIG. 2 is a cross-sectional view of a modification.

Referring now to the drawing, there is shown in FIG. 1 a sealed enclosure 1 which may be made of glass, metal, or any other substantially gas-tight material which is opaque to radiation-of a wavelength of 1216 A. One wall of the enclosure contains a window 2 which is substantially transparent to far ultraviolet light or radiation with a wavelength of about 1216 A. A suitable window material for this radiation is lithium fluoride. The opposite wall of the enclosure has been employed for introducing a desired gas into the enclosure and for thereafter sealing-off the enclosure, which is shown by reference numeral 3. Within the enclosure is contained a quantity of radioactive tn'tium gas 4 at a pressure of, for example, one atmosphere, which is not at all critical as the pressure or the amount of the tritium present merely determines the intensity of the light output.

Radioactive tritium, a heavy isotope of hydrogen, has a half-life of 12.26 years. During the decay of the radioactive tritium, only low-energy beta particles are emitted, with amaximum energy of about 18 kev. and an average energy of about 5 kev., and no gamma radiations. About -2,990,491 Patented June 27, 1961 ice 3X10 beta particles are produced per second for each curie of the radioactive material, and each cubic centimeter of this gas at atmospheric pressure contains about having a far ultraviolet ligh't tr t n spa g e of transmr mg ra 1a lon wilh" a wavele 2.7 curies. The beta particles emitted by the radioactive tritium dissociate hydrogen and tritium molecules present in the enclosure and the hydrogen atoms thereby produced in addition to tritium atoms in the enclosure are excited by the beta particles to emit a series of spectral lines which include the desired radiation of about 1216 A. The hydrogen present in the gas either arises from dissociated tritium, or a quantity of non-radioactive hydrogen may be mixed in with the radioactive tritium. The proportions of this mixture are not critical and will depend upon the pressures desired of the gas within the enclosure.

The emitted 1216 A. radiation traverses the transparent window portion 2 and is thus made available on the outside of the enclosure, which has been indicated by dotted lines designated by reference numeral 5.-

The far ultraviolet light source thus described has several important advantages. Firstly, the device is selfcontained and is characterized by an extremely simple structure which nevertheless has a very long life and is very stable. Further, a most important feature is that no spurious radiation outside of the light range, visible or near-visible, emanates from the source, that is, no beta particles emanate from the source nor is there any gamma emission produced during the decaying process. No betas are emitted because those produced by the decay of the radioactive tritium which are not absorbed by the gas itself are, because they are of such low-level energy, readily absorbed in the walls and the window constituting the enclosure for the gas. This is an important consideration in systems employing such sources because spurious beta particles or gamma radiation emanating from the source would be counted by the Geiger counter of the system, and so misleading counting rates would result as these forms of radiation will not be attenuated by water molecules.

FIG. 2 shows a modification of the device of the invention in which a heated member, shown as a filament 6 capable of being heated by the passage of current therethrough, is mounted within the gas-filled enclosure. The presence of a heated member causes dissociation of molecular hydrogen or molecular tritium present in the euclosure and thus makes available more atomic hydrogen or tritium for excitation by the emitted beta particles from the decaying tritium. The filament may be made of tungsten or of like conventional filament material. In addition, the far ultraviolet opaque wall portions 1 may be provided with a metallized surface 7 serving as a reflector to increase the intensity of the light output in the desired direction. A suitable material for this purpose is platinum.

While we have described our invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claims.

What isclaimed is:

l janu ltr avjglet light source cgmprising an enclosure ortiilujcapable 'fi of 1216 A., and radioactive 'theeh'closure." *ZT-AsQurce as claimed in claim 1 in which nonradioactive hydrogen gas is admixed with the tritium gas.

3. A far ultraviolet light source for generating radiation with a wavelength of about 1216 A., comprising a sealed enclosure containing a far ultraviolet light-opaque portion and a far ultraviolet light-transparent window portion capable of transmitting 1216 A. radiation, and radio active tritium gas within the enclosure.

4. A source as set forth in claim 3 wherein the windoi is of lithium fluoride.

5. A source as set forth in claim 3 including means for 7. A source as set forth in claim 3 wherein a reflecting cooperating with the radioactive tritium to provide a subcoating is provided on the opaque portion of the enclosure. stance selected from the group consisting of atomic hydrogen and atomic tritium within the enclosure. References Cited m the me of am patent 6. A source as set forth in claim 5 wherein the said 6 UNITED STATES PATENTS means comprise a filament that can be heated within the 2,699,515 Williams Ian. 11, 1955 enclosure. 2,804,573 Arrol Aug.27. 1957 

1. A FAR ULTRAVIOLET LIGHT SOURCE COMPRISING AN ENCLOSURE HAVING A FAR ULTRAVIOLET LIGHT-TRANSPARENT PORTION CAPABLE OF TRANSMITTING RADIATION WITH A WAVELENGTH OF 1216 A., AND RADIOACTIVE TRITIUM GAS WITHIN THE ENCLOSURE. 